Satellite digital audio radio services (SDARS) broadcast audio programming directly from a satellite to an end user's radio receiver, so that a typical SDARS broadcast reaches an extensive, diverse, geographical region. In order to ensure high quality, uninterrupted transmission in all the reception regions reached by the broadcast, SDARS providers typically complement their satellite broadcast with gap-filling rebroadcasts using terrestrial stations located in regions having poor or no satellite reception, such as cities with tall buildings. The signals broadcast from the satellite and by the terrestrial stations contain the same audio data, and are typically on adjacent frequencies but use different modulation techniques. The terrestrial signals are also typically broadcast at significantly higher signal strength, primarily because terrestrial stations have easy access to electrical power while satellites are limited to the electrical power available from their solar panels.
An exemplary SDARS system is the service provided by Sirius Radio Systems of New York, N.Y., which broadcasts over 100 channels of audio programming directly from satellites to users equipped with appropriate receivers.
FIG. 1 shows the relative frequencies and power levels of the signals in the Sirius system. Two geo-synchronous satellites transmit S band (2.3 GHz), time division multiplexed (TDM) signals directly to the end user's receiver, which is typically a mobile receiver in an automobile or a truck. In regions with poor satellite reception, terrestrial repeater stations broadcast a coded orthogonal frequency division multiplexed (COFDM) signal containing the same audio data as that broadcast in the satellite signals. The terrestrial COFDM signals are broadcast at an S band frequency, lying between the frequencies of the two satellite TDM signals, and at a significantly higher power level.
FIG. 2 shows a schematic diagram of a prior art, digital radio receiver designed to receive and decode the audio channels contained in the Sirius system signals. The receiver 10 has two decoding circuits 12 and 14, one for receiving TDM signals directly from the satellites and one for receiving COFDM signals. The TDM decoding circuit has a TDM antenna 16 for receiving the signal, which is then amplified by TDM variable gain amplifier (VGA) 18. The amplified signal is digitized by a TDM analogue-to-digital converter (ADC) 20. The digitized TDM signals are down-converted by TDM digital-down-converter (DDC) 22, before being demodulated. In the Sirius system, there are two geo-synchronous satellites visible at any one time, so there are two TDM demodulators 24 and 26, one for handling each of the signals.
The ADC 20, which is typically a 10 bit device with a usable dynamic range of about 52 dB, plays an important role in digital radio reception. As long as the digitized signal is an accurate representation of the incoming analogue signal, digital filtering techniques make it possible to extract very weak signals, such as those received from a satellite, even in the presence of a significant amount of noise. Accurate digitization requires that the incoming signal is amplified sufficiently to fill as much of the ADC's dynamic range as possible. It is, however, also very important not to over amplify the incoming signal since, when the ADC is overdriven and overflows, a small signal in a noisy background can be completely lost. This happens because the ADC simply truncates any excess signal.
The appropriate gain setting of VGA 18 that amplifies the incoming signal to the optimal level for the ADC is controlled by the TDM automatic gain control (AGC) 28. The AGC monitors the demodulated TDM signals, and uses the stronger of the two demodulated TDM signals to set the gain of VGA 18 so that the portion of the received signal containing the best TDM signal is amplified appropriately, and a constant level output is obtained.
Any available COFDM signal is demodulated using a parallel COFDM decoding circuit 14, having COFDM antenna 30, VGA 32, ADC 34, COFDM 36, COFDM demodulator 38, and COFDM AGC 40. All the demodulated signals are summed together in sum module 42.
In prior art receivers designed for the Sirius system, the front end of both the TDM and the COFDM decoding circuits contain substantially identical components, i.e., the TDM and COFDM antennas 16 and 30, VGAs 18 and 32 and ADCs 20 and 34 are the same as each other. In order to reduce the power requirements and the cost of receivers, it is highly desirable to have a receiver with only one front-end, i.e., only one antenna 16, one VGA 18 and one analogue-to-digital converter (ADC) 20, as shown schematically in FIG. 3.
Practical implementation of a single front-end circuit of the type shown in FIG. 3 is not, however, simple. A major problem in such a circuit is that the VGA gain settings for the two types of signal may be incompatible with each other. This causes difficulties if the VGA gain is controlled using a simple, two-state AGC 43, with one state to optimize the gain for a COFDM signal and one state to optimize the VGA gain for a TDM signal. In such a system, a VGA gain that is optimal for the weak TDM signals from the satellite will typically over-amplify the incoming COFDM signal from the terrestrial stations, resulting in the COFDM signal over-flowing the ADC's dynamic range. This over-flow of the ADC's dynamic range means that the demodulated COFDM audio data is of very poor quality, and may even be non-existent. The receiver may also be blocked from reception of the TDM signal.
Similarly, if the VGA gain setting is optimal for the ADC to digitize the portion of the signal containing the stronger, COFDM signal, the portion of the signal containing the TDM signal will be under-amplified, and poorly digitized by the ADC. The result is that if the receiver does lock on to a terrestrial COFDM signal, it may stay locked onto the terrestrial signal, even if there is a better satellite signal available.
In order to achieve the highly desirable power and cost savings that a single VGA and ADC would provide, it is necessary to have an automatic gain control that can adjust the VGA gain in a way that makes it possible to use the best available signal, and not to be blinded to the availability of a better signal by either under or over amplifying any portion of the signal with respect to the ADC dynamic range.